The invention relates to a circuit arrangement for reducing the settling time of logarithmic amplifiers to which photoelectronic components are connected.
Logarithmic amplifiers having currents supplied by photodiodes generally have switch-on problems when the photodiode currents are very small. This is becuase settling processes occurring during switch-on have the effect, in conjunction with the rectifier characteristics of the logarithmic diodes which are connected to these amplifiers, that voltage peaks fully reach the photodiodes in one polarity; but in the other polarity the logarithmic diodes completely block the peaks. As a result, the photodiode is charged up in the wrong direction. Recharging can only be done by the very small photocurrent since the photodiodes and the logarithmic diodes block up to a very high voltage. In most cases, the photodiodes have a considerable capacitance so that a long, undesirable time elapses for the recharging. Due to this "false charge", the output of the amplifier is latched up to a particular but undesired value for a certain time after switch-on. This is why such a circuit arrangement does not amplify the current detected and indicates a small amount of light and does not work normally unitl the "false charge" across the photoelectronic component has been removed.
To reduce the effect of this switch-on time, it has ben proposed in German Offenlegungsschrift No. 2,833,217 (incorporated herein by reference) to supply the input of an amplifier following the photolectronic component with an auxiliary voltage via a differentiating section.
Circuit arrangements of this type must meet extreme requirements with respect to residual currents and are therefore of elaborate technical construction or present problems for reasons of stability. In addition, correct indication of the current measured by the arrangement is not possible immediately after switching on the operating voltage.
To acheive a simpler construction of such a circuit arrangement in which a differential amplifier is connected together with a photodiode, and its output is fed back via a logarithmic diode to the inverting input, it is proposed in German Offenlegungsschrift No. 3,113,220 (incorporated herein by reference) "to reduce" the feedback current flowing via the logarithmic diode in the settling phase of the input stage by connecting the output of the differential amplifier to ground potential during a limited switched-on period.
It is a disadvantage also in this case that no indication can be derived from the exposure control circuit immediately after the operating voltage is applied to the circuit arrangement and then, despite the reduction of the feedback current flowing via the logarithmic diode, a settling phase can occur until the operating point is reached, which again causes the same problem of false charging.